Vacuum Distillation Process

ABSTRACT

The present invention relates to a process and apparatus for the production of diesel fuel from feedstocks containing fatty acids, glycerated fatty acids, and glycerin by vacuum distillation followed by esterification. Specifically, the present invention relates to the production of Renewable Diesel having low glycerin, water, and sulfur content. Operation of the distillation system enables production of esters including biodiesel and other biofuels in an economically advantageous manner. The vacuum distillation system is optionally located upstream of an esterification unit or other biodiesel production facility for improvement in production economy.

This application claims priority under 35 U.S.C. 119(e) to U.S.provisional application 60/962,690, filed Jul. 31, 2007, the contents ofwhich are incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the field of refining and/or producinghydrocarbons such as biofuels by the method of vacuum distillation.

BACKGROUND

In an effort to partially replace dependence on petroleum-based diesel,vegetable oils have been directly added to diesel fuel. These vegetableoils are composed mainly of triglycerides, and often contain smallamounts (typically between 1 and 10% by weight) of free fatty acids.Some vegetable oils may also contain small amounts (typically less thana few percent by weight) of mono- and di-glycerides.

Triglycerides are esters of glycerol, CH₂(OH)CH(OH)CH₂(OH), and threefatty acids. Fatty acids are, in turn, aliphatic compounds containing 4to 24 carbon atoms and having a terminal carboxyl group. Diglyceridesare esters of glycerol and two fatty acids, and monoglycerides areesters of glycerol and one fatty acid. Naturally occurring fatty acids,with only minor exceptions, have an even number of carbon atoms and, ifany unsaturation is present, the first double bond is generally locatedbetween the ninth and tenth carbon atoms. The characteristics of thetriglyceride are influenced by the nature of their fatty acid residues.

The production of alkyl esters of glycerides by transesterification is aknown process. However, transesterification suffers in that the reactiongenerally requires the addition of an acid or base catalyst which mustbe neutralized after the reaction thereby generating salts and soaps. Inaddition, while transesterification results in the separation of fattyacid esters from triglycerides, it also results in the production ofglycerin, which must then be separated from the fatty acid esters,glycerin, excess alcohol, salts, and soaps.

The production of alkyl esters of fatty acids by acid catalyzedesterification is also known. However, the use a strong acid, such assulfuric acid, typically leads to higher sulfur content in the resultingester as the acid reacts with the double bonds in the fatty acid chains.In addition, conversion of the esterification reaction is limited byequilibrium constraints such that either excessive time and equipmentsize are required or less conversion needs to be accepted. In an effortto overcome some of the problems associated with transesterification,several attempts have been made to employ esterification between fattyacids and alcohols. In these processes, fatty acids are prepared fromtriglycerides by hydrolysis, followed by catalyzed esterification of thefatty acids with an alcohol, preferably methanol. While this practice ispracticed in the production of fatty alcohols and fatty acid esters, asdescribed in U.S. Pat. No. 5,536,856 (Harrison et al), it has not beenpracticed in the production of Renewable Diesel.

Vacuum distillation of hydrocarbons is a refining process that can beutilized to minimize thermal cracking of heavier fractions ofhydrocarbons and obtain lighter desired products. Distilling thesematerials under vacuum, i.e., lower pressure, decreases the boilingtemperature of the various hydrocarbon fractions in the feed andtherefore minimizes thermal cracking of these fractions.

In conventional vacuum distillation systems, distillation is carried outin a vacuum column under pressures typically in the range of 25 to 100millimeters of mercury (mmHg). It is important in such systems to reducepressure as much as possible to improve vaporization. Vaporization isenhanced by various methods such as the addition of steam at the furnaceinlet and at the bottom of the vacuum distillation column. Vacuum iscreated and maintained using cooling water condensers and steam drivenejectors. The size and number of ejectors and condensers used isdetermined by the vacuum needed and the quantity and quality of vaporshandled. While vacuum distillation is practiced in the production ofpetroleum-based products, it has not been practiced in the production ofRenewable Diesel in a continuous process combined with esterification ina reactive distillation process.

Accordingly, one object of the present invention is to utilize vacuumdistillation in combination with reactive distillation duringesterification to produce a product that meets either ASTM D396 orASTMD975 specifications or both.

SUMMARY

The present invention provides a vacuum distillation system and methodutilizing high free fatty acid feedstock. Operation of the distillationsystem enables production of esters including biodiesel and otherbiofuels in an economically advantageous manner. The vacuum distillationsystem is optionally located upstream of an esterification unit or otherbiodiesel production facility for improvement in production economy.

In one embodiment, the invention is a process for preparing RenewableDiesel comprising the step of vacuum distillation of oil of vegetableand/or animal origin followed by the step of esterification to yielddiesel fuel according to either ASTM D396 or ASTM D975 or both. In apreferred embodiment, the feedstock for the process is an oil ofvegetable and/or animal origin which is selected from Palm Fatty AcidDistillate (PFAD), Palm Acid Oil (PAO), Acid Oil (Acidulated SoapStock), and mixtures thereof. In one embodiment, the feedstock oilfurther comprises up to about 20% pre-split tallow and/or up to about 5%tall oil fatty acid. Alternatively, in one embodiment, the feedstock oilis essentially free of tallow or poultry fat.

In one embodiment, esterification is carried out by reactivedistillation using a solid catalyst. Preferably, the solid catalyst isan ion exchange resin catalyst comprising —SO3H or —CO2H functionalgroups. Preferably, the step of esterification is performed with analcohol selected from methanol, t-butanol, isobutanol, or a mixturethereof.

In one embodiment, a fat splitter may be integrated into the process,optionally between the vacuum distillation unit and the esterificationunit.

In one embodiment, the Renewable Diesel is high value and low acidity.In a preferred embodiment, the process is carried out on an industrialscale. Preferably, the process is continuous.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of the present reaction for the preparationof low acidity Renewable Diesel by vacuum distillation followed byreactive distillation esterification.

FIG. 2 shows another embodiment of the present reaction for thepreparation of low acidity Renewable Diesel by vacuum distillationfollowed by reactive distillation esterification.

FIG. 3 shows a process flow diagram model according to the invention,providing details of various embodiments.

DETAILED DESCRIPTION

The present invention provides a process for the production of RenewableDiesel fuels having low acidity, low glycerin, and low sulfur content,from oils of animal and vegetable origin. Renewable Diesel is a materialderived from animal or vegetable origin which meets ASTM D396 and/orASTM D975 specifications. The process of the present invention employs acombination of vacuum distillation and, if desired, reactivedistillation esterification to produce fuels that meet either or both ofthese standards and therefore qualify as Renewable Diesel.

The animal or vegetable oil feedstocks for use according to theinvention include, but are not limited to, fatty acids such as decanoicacid, dodecanoic acid, tetradecanoic acid, hexadecanoic, acid,octadecanoic acid, octadecenoic acid, linoleic acid, eicosanoic acid,isostearic acid and the like, as well as mixtures of two or more thereofas well as oils described as coconut oil, rape seed oil, and palm oils,tall oils, lard, bacon grease, yellow grease, tallow and fish oils.Additional oils may be sourced from whale oil and poultry fat, as wellas corn, palm kernel, soybean, olive, sesame, and any other oils ofanimal or vegetal origin not explicitly identified herein. If desired,such mixtures of acids can be subjected to distillation to remove lowerboiling acids having a lower boiling point than a chosen temperature(e.g. C₈ to C₁₀ acids) and thus produce a “topped” mixture of acids.Optionally, the mixtures can be distilled to remove higher boiling acidshaving a boiling point higher than a second chosen temperature (e.g. C₂₂⁺ acids) and thus produce a “tailed” mixture of acids. Additionally,both lower and higher boiling acids may be removed and thus produce a“topped and tailed” mixture of acids. Such fatty acid mixtures may alsocontain ethylenically unsaturated acids such as oleic acid. Suchmixtures may also contain fatty acid esters. In a preferred embodiment,the feedstock for the process is an oil of vegetable and/or animalorigin which is selected from Palm Fatty Acid Distillate (PFAD), PalmAcid Oil (PAO), Acid Oil (Acidulated Soap Stock), and mixtures thereof.In one embodiment, the feedstock oil further comprises up to about 20%pre-split tallow and/or up to about 5% tall oil fatty acid. For example,in one embodiment, about 115 MMlb/yr of pre-split tallow may be used.Use of tall oil fatty acid is envisioned as a cold flow improvingblendstock. Alternatively, in one embodiment, the feedstock oil isessentially free of tallow or poultry fat. By essentially free is meantthat tallow or poultry fat are not introduced into the feedstock, or inthe event of residual contamination from processing of various batches,are not introduced in quantities that affect the processing parametersof the feedstock.

In one embodiment, the purpose of vacuum distillation is to distill allor nearly all fatty acids and those triglycerides not needed for steamgeneration while leaving a pitch that contains high-boiling material. Inone embodiment, additional beneficial effects may be obtained at higherpressures and temperatures, such as additional fat splitting. In apreferred embodiment, four cuts are made during distillation: 1) water,glycerin, fatty acids, and low boiling material; 2) fatty acids andmonoglycerides; 3) tri- and di-glycerides; and 4) pitch. In oneembodiment, five cuts are made during distillation: 1) water and lowboiling material; 2) glycerin and fatty acids; 3) fatty acids andmonoglycerides; 4) tri- and di-glycerides; and 5) pitch.

In a preferred embodiment, steam from a pre-existing natural gas turbineor coal power plant is used to reduce costs of construction. In apreferred embodiment, pitch or waste organics generated during vacuumdistillation are sold to a third-party to improve the overall economicequation for the process.

Reactive Distillation Esterification refers to a process taking place ina column so designed such that the vapor stream of the more volatile ofthe two components, (i.e. the more volatile of the vacuum distillationproduct component and the alcohol component), flows countercurrent tothe less volatile component such that the vapor stream carries awaywater produced in the esterification reaction, while advantageously notcarrying away a significant quantity of the less volatile component. Forthis reason it is essential that the boiling point of the vapor mixtureexiting the esterification reactor, or of the highest boiling compoundpresent in that vapor mixture, be significantly lower, at the pressureprevailing in the uppermost stage of the esterification reactor, thanthe boiling point at that pressure of either of the less volatile one ofthe two components. By the phrase “significantly lower” is meant thatthe boiling point difference shall be at least about 20° C., andpreferably at least about 25° C., at the relevant operating pressure ofthe column. In practice, the more volatile component of the two willfrequently be the alcohol component. For example, methanol will be themore volatile component in the production from fatty acid mixturesobtained by the hydrolysis of triglycerides of methyl fatty acid estermixtures for subsequent processing, for example for production ofdetergent alcohols by ester hydrogenation.

In one aspect of the present invention, Renewable Diesel fuels preparedaccording to the present invention are provided. Sulfur content of theRenewable Diesel fuel is one of many parameters of interest forcommercial use. Sulfur is typically present as a result of the use ofsulfuric acid catalysts, and can result in increased engine wear anddeposits. Additionally, environmental concerns dictate a desired lowsulfur content in the Renewable Diesel fuel. Preferably, RenewableDiesels prepared according the methods provided herein have a sulfurcontent (as measured by ASTM test method D5453) of less than 500 ppm,more preferably less than 200 ppm, less than 100 ppm, less than 50 ppm,less than 25 ppm, less than 10 ppm, and most preferably less than 5 ppm.

The cetane number (i.e., the measure of the ignition quality of thefuel, as measured by ASTM test methods D976 or D4737) is preferablygreater than 47, more preferably greater than 50, and most preferablygreater than 55.

Cloud points are defined as the temperature at which a cloud or haze ofcrystals appears in the fuel. Cloud points determine the climate andseason in which the Renewable Diesel fuel may be used. Preferably thecloud point of the Renewable Diesel is less than 0° C., more preferablyless than −5° C., less than −10° C., less than −15° C., less than −20°C., less than −25° C., less than −30° C., less than −35° C., less than−40° C., and most preferably, less than −45° C.

Total free glycerin in the Renewable Diesel is preferably less than0.03% by weight, more preferably less than 0.20% by weight, less than0.018% by weight, less than 0.016% by weight, and most preferably, lessthan 0.015% by weight. Total glycerin present in the Renewable Dieselfuel is preferably less than 0.25% by weight, more preferably less than0.24% by weight, less than 0.23% by weight, less than 0.22% by weight,0.21% by weight, and most preferably, less than 0.20% by weight.

Residual methanol in the Renewable Diesel is desired to be minimized,and is preferably less than 0.2% by weight, more preferably less than0.18% by weight, and most preferably less than 0.15% by weight.

Water content in the Renewable Diesel fuel produced according thepresent invention is preferably less than 500 ppm, preferably less than450 ppm, more preferably less than 400 ppm and most preferably less than300 ppm.

It can be important to define a minimum viscosity of the RenewableDiesel fuel because of power loss due to injection pump and injectorleakage. Preferably, the viscosity of the Renewable Diesel fuel isbetween 1.0 and 50 mm²/s, more preferably between 1.3 and 15.0 mm²/s,even more preferably between 1.3 and 2.1 mm²/s.

In one embodiment, the Renewable Diesel is produced on an industrialscale. Global biodiesel production is estimated at several million tonsper year. In a preferred embodiment, production occurs from 500 kg ormore of feedstock per day. Alternatively, production may occur onbatches or continuous feeds of 1,000 kg, 5,000 kg, 10,000 kg or morefeedstock per day. Alternatively, vacuum distillation may occur on ascale of about 200 tons per day, while esterification may occur on ascale of about 300 tons per day. In one embodiment, a fat splitter withcapacity of, for example, about 100 tons per day or about 150 tons perday, may be integrated into the process, optionally between the vacuumdistillation unit and the esterification unit. Additionally, a properlyscaled glycerin concentration unit assuming feedstock with about 30%free fatty acid may be incorporated into the process. Glycerin recoveryis envisioned to account for glycerin carryover from the vacuumdistillation process, if present.

Referring now to FIG. 1, there is provided an embodiment of a processfor the preparation of Renewable Diesel from animal and vegetable oils.Feed Stream 1 contains liquids derived from animal and vegetablesources. Such liquids may contain fatty acids, glycerides of fattyacids, esters, alcohols, and other hydrocarbons. Feed stream 1 couldalso contain petroleum derived hydrocarbons. Feed stream 1 is fed to avacuum distillation unit 2.

Vacuum distillation unit 2 may or may not be equipped to contain athermal oxidizer for management of tank vapors as well as an emergencyMeOH scrubber able to operate without plant power. Preferably, vacuumdistillation unit 2 can operate without any fired heaters, but a steamheating and/or hot oil system may optionally be included to allow fordistillation at higher temperatures. Vacuum distillation unit 2, and anycoupled separation device such as a glycerin condensing unit or a fattyacid splitter, is operated in order to, at a minimum, provide free fattyacids to the reactive distillation esterification unit. Vacuumdistillation unit 2 and any coupled device therewith incorporated mayproduce a product 3 consisting of lower boiling hydrocarbons, CO, CO₂,hydrogen, and water and liquid product 4. Liquid product 4 of vacuumdistillation unit 2 may or may not meet all specifications of ASTM D396and/or D975 at this point, but liquid product 4 may meet thedistillation and flash point ranges are near as possible.

In one embodiment, liquid product 4 is fed to Reactive DistillationEsterification Unit 6. Reactive Distillation Esterification Unit 6 isalso fed with an alcohol stream 5. Within the Reactive DistillationEsterification Unit 6, acidic components in the liquid product 4 arereacted with the alcohol from stream 5 and converted to esters product7. Water of reaction and alcohol are also separated so that excessalcohol used in the reaction can be recycled.

Referring to FIG. 2, the same process is contemplated with thedifference being the feeding of stream 8 along with the feedstock instream 1. Stream 8 contains water or steam. Feeding steam or water asstream 8 along with feed stream 1 is intended to help maximize theoutput of liquid product 4. In one embodiment, FIG. 2 corresponds in allother regards to FIG. 1.

Referring to FIG. 3, one possible embodiment according to the inventionis provided, including a vacuum distillation unit and an esterificationunit.

It will be understood by those skilled in the art that the drawings arediagrammatic and that further items of equipment such as reflux drums,pumps, vacuum pumps, temperature sensors, pressure sensors, pressurerelief valves, control valves, flow controllers, level controllers,holding tanks, storage tanks, and the like may be required in acommercial plant. The provision of such ancillary items of equipmentforms no part of the present invention and is in accordance withconventional chemical engineering practice.

Modifications and variations of the present invention relating to theselection of reactors, feedstocks, alcohols and catalysts will beobvious to those skilled in the art from the foregoing detaileddescription of the invention. Such modifications and variations areintended to come within the scope of the appended claims. All numericalvalues are understood to be prefaced by the term “about” whereappropriate. All references cited herein are hereby incorporated byreference in their entirety.

The process and apparatus of the invention can be used in biodieselrefining, and in petrochemical and other industries where vacuumprocessing of liquid products is required. It is possible toeconomically integrate the invention process into conventional vacuumdistillation systems. It should be noted that various changes andamendments can be made in the details within the scope of the claims setforth below without departing from the spirit of the claimed invention.It should therefore be understood that the claimed invention should notbe limited to the specific details shown and described.

1. A process for preparing Renewable Diesel comprising the step ofvacuum distillation of oil of vegetable and/or animal origin followed bythe step of esterification to yield diesel fuel according to either ASTMD396 or ASTM D975 or both.
 2. The process of claim 1, wherein the oil ofvegetable and/or animal origin is selected from Palm Fatty AcidDistillate (PFAD), Palm Acid Oil (PAO), Acid Oil (Acidulated SoapStock), and mixtures thereof.
 3. The process of claim 2, wherein the oilfurther comprises up to about 20% pre-split tallow and/or up to about 5%tall oil fatty acid.
 4. The process of claim 2, wherein the oil isessentially free of tallow or poultry fat.
 5. The process of claim 1,wherein esterification is carried out by reactive distillation using asolid catalyst.
 6. The process of claim 5, wherein the solid catalyst isan ion exchange resin catalyst comprising —SO₃H or —CO₂H functionalgroups.
 7. The process of claim 1, wherein the process is carried out onan industrial scale.
 8. The process of claim 1, wherein the RenewableDiesel is high value and low acidity.
 9. The process of claim 1, whereinthe process is continuous.
 10. The process of claim 1, wherein the stepof esterification is performed with an alcohol selected from methanol,t-butanol, isobutanol, or a mixture thereof.